1. Technical Field
This invention relates generally to an optical pulse generator and more particularly to a data encoded optical pulse generator for generating soliton pulses.
2. Discussion of Related Art
A useful measure of the performance characteristics of a digital optical data communication system is the well-known "rate-length product", i.e., the product of the system data rate and the length of transmission. It is often a design goal to achieve the highest data rate through the longest repeaterless length and consequently the highest rate-length product.
Components used in long distance optical data communication systems for producing a high rate-length product typically include: a light source such as a laser diode; a high speed modulator which modulates the light source at the system bit rate, usually by amplitude modulating the light produced by the laser; a low loss, low dispersion fiber medium; a photodetector such as a p-i-n photodiode or an avalanche photodiode having a high speed response for detecting the transmitted optical signals; and a receiver coupled to the photodetector for amplifying and decoding the received optical signals. Components such as optical amplifiers and repeaters can further extend the transmission length and increase the rate-length product.
The system rate-length product is a function of the transmission format as well as of the hardware components used. In current optical communication systems, data is transmitted in non-return-to-zero (NRZ) format, with ones and zeros represented by the presence or absence of light in a given time slot. This format is typically implemented by using a laser to generate a CW light beam, then modulating the light beam with an electro-optic modulator. The modulator may be a separate semiconductor device. Recently, the laser and modulator have been fabricated on a single chip, resulting in an integrated transmitter for NRZ communications systems.
The rate-length product attainable with the NRZ transmission format is ultimately limited by dispersion in the optical fiber. To reach very high data rates, communications links employing soliton pulses have been proposed. Solitons are optical pulses which take advantage of the nonlinearity of the fiber to maintain pulse shape during transmission. Soliton pulses can be transmitter over long lengths of fiber at rates of 10 Gb/s and higher. The soliton pulse width is less than the width of the time slot and is thus transmitted in return to zero (RZ) format; i.e., the amplitude of the light returns to zero during each time slot. An RZ format is also desirable for systems employing optical time-division-multiplexing and demultiplexing.
Despite recent advances in the development of optical transmitters, a need exists for a transmitter which produces soliton pulses and is wavelength tunable, compact, manufacturable and relatively inexpensive to operate for producing RZ encoded data. One approach is to use a gain-switched laser to generate pulses, then encode data onto the pulses with a modulator. However, gainswitching produces chirp, which causes significant penalties in transmission due to fiber dispersion, and may also degrade the performance of a soliton system. Another approach is to use a mode-locked laser to generate pulses and again encode data with a modulator. However, monolithic mode-locked lasers operate only at fixed frequencies which are determined by their cavity length. The fabrication of devices with a desired operating frequency may be difficult, especially if the laser is to be integrated with the modulator for data encoding. External-cavity mode-locked lasers offer more flexibility in operation frequency, but are bulky and sensitive to the environment, making them unsuitable for practical applications.
Integrated laser-modulators previously suitable for NRZ data transmitters can be operated as transmitters for producing soliton pulses. In such case the laser is operated CW and the modulator is driven with an RF sinusoid, resulting in a time-varying transmission which converts the CW laser light to pulses. This type of pulse source is simple, compact, frequency and wavelength tunable, and possesses spectral characteristics suitable for long distance transmissions. A soliton transmitter based on this pulse source, consisting of a laser integrated with two modulators, has been demonstrated. In this approach, the first modulator is used to convert CW laser light to pulses and the second modulator is used to encode data. While this technique is attractive, the integration of a laser and two modulators requiring two high-speed contacts is difficult. A simpler device would be preferable.